Patent GB-726550-A (GB)
Title: Improved fluid pressure servo system for propeller pitch control
IPC: B64C11/42
Filed: 1953-02-02
Abstract: 726,550. Fluid-pressure servomotor-control systems. GENERAL MOTORS CORPORATION. Feb. 2, 1953 [Feb. 2, 1952], No. 2859/53. Class 135. A variable pitch propeller adjustable by a fluid-pressure servomotor has fluid-pressure effected stop means for limiting adjustment at either of two positions, e.g. minimum pitch for flight and a lower minimum pitch for ground testing, and fluid-pressure actuated means for determining-the pitch at which said stop limits adjustment. A servomotor 80 for adjusting the blades 90 is controlled, by a valve 70 which is fed with pressure fluid by a pump 10 and reciprocated by solenoids 279, 281 under the control of a governor whose datum is set by a lever 300. The pump 10 also supplies through a conduit 20 an overspeed valve 30, an underspeed valve 40, and a distributer valve 50. These valves are rotatable with the propeller so that over-speeding causes the valve member 32 of the valve 30 to move upwardly against a spring 236 to which it is .connected by a lever 232 pivoted at .234 .and supply pressure fluid through a conduit 5 to a selector valve 60 which directs the fluid through a 'conduit' 7 to increase the pitch of the propeller, a conduit 8 being connected to drain through the valve 60, a conduit 4 and port 39. Similarly, a fall below a predetermined speed causes downward movement of the valve member 42 of the valve 40 under the influence of a spring 246 resulting in pressure fluid being supplied to the conduit 8 and exhausted from the conduit 7 to reduce pitch. Driven from the propeller blades through gearing 88, 94 is a feed-back shaft consisting of a part 96 having a sliding connection with a screw-threaded part 216 upon which are threaded carriages 214, 230, 240 and a yoke 250. The part 216 of the feed-back shaft is movable axially by pressure fluid applied to a piston 218, and in its left-hand position, as shown, prevents the propeller blades from being adjusted to a lower pitch than the minimum for flight. This is effected by engagement of a raised portion of the carriage 230 with a roller 238 on the lever 232 due to leftward movement of the carriage as the pitch is reduced causing the valve 30 to be raised to over-ride the valve 70 in control of the servomotor. At the same time, a raised portion of the carriage 240 engages a roller 248 on a lever 242 to prevent the valve 40 from acting as an underspeed valve. To select a lower pitch for testing on the ground, the lever 300 must first be moved from the normal governed speed range to a range for blade angle control, i.e. to the left. Such movement renders the valve 70 inoperative and causes angular adjustment of a valve member 310, due to linkage 302, 304, 306, to place a conduit 77 in communication with the pressure fluid supply to depress the valve member 62 of the selector valve 60 to isolate the conduits 7, 8 from the overspeed and underspeed valves 30, 40 and place them in communication with conduits 18, 19 controlled by the distributer valve 50, the valve member 52 of which is adjusted by the lever 300 through linkage 302 and a lever 264 which engages a cam surface 254 of a lever 256 pivoted at 258. Movement of the servomotor 80 in response to actuation of the valve 50 causes the valve member 52 to be returned to its closed position through rocking of the lever 264 by movement of the yoke along the shaft 216. At a certain low angle of pitch adjustment, a pin 212 mounted on the carrier 214 and coacting with a cam slot 211 in a lever 207 which is connected to a valve member 202 causes the latter to uncover ports such that fluid pressure acting on the upper end of a sleeve 168 is released allowing a spring 194 to raise the sleeve and open ports which cause fluid pressure to be applied to the left-hand side of the piston 218 and released from the righthand side, thereby causing the shaft 216 to move to the right. The lever 300 may then be returned to the governed speed range, causing the valve member 310 to vent the conduit 77 and allow the valve member 62 to be raised by a spring 71, when a lower propeller pitch may be obtained before the carriage 230 causes the valve 30 to take control. When a comparatively high pitch is reached during subsequent running, e.g. during take-off, the pin 212 resets the valve member 202 to cause the piston 218 to be returned to the left-hand position. Pressure fluid from the pump 10 is reduced in pressure by a valve 100 before passing to the valve member 310 through a passage 114 and to the valve member 202 through a passage 115. Specifications 544,580 and 545,520, [Group XXXIII], are referred to.